Most presently-constructed helicopters are fitted with one or more turbine engines, referred to as a turbine engine power plant, generally using a free turbine. A speed-reducing gearbox, also referred to as a main transmission gearbox (MGB) serves to make the driving connection between the engine unit and the rotary wing, specifically the rotor driving the blades.
By way of example, patent EP 0 816 226 discloses a flight indicator for an aircraft that is designed to show the power margin available on at least one engine of the aircraft as a function of flying conditions. The indicator uses signals from sensors suitable for delivering information concerning various monitoring parameters of the engine, processor means for processing the information coming from said sensors, and display means that present the processed information on a display screen, which information relates to that one of the parameters, from amongst said plurality of engine monitoring parameters, for which the current value is the closest to a defined limit value for said parameter. The limit values are established in real time while taking account of ambient pressure and temperature.
The above-mentioned flight indicator thus identifies, in relative values, that one of the essential engine monitoring parameters that is closest to its limit. The information relating to the limit to be complied with is then grouped together in a single display serving firstly to summarize the information and to present the pilot solely with the result of this summary, thereby simplifying the pilot's task, and secondly to save space on the instrument panel.
The flight indicator thus includes various sensors that relate to different parameters that need to be monitored and that are suitable for delivering information corresponding to those various parameters to a computer that processes said information. Display means then present on a display screen information that relates to a so-called “limit” parameter selected from amongst said engine monitoring parameters as being the parameter for which the current value is the closest to the corresponding limit value. Such a flight indicator is referred to as a first-limit indicator or FLI for short.
In the specific example of a rotary wing aircraft, e.g. a helicopter, having at least one turbine engine and at least one rotor, the particular monitoring parameters are associated with said turbine engine and with the main gearbox.
Present-day flight indicators of the FLI kind are nevertheless not sufficient under certain circumstances concerning the information they show the pilot. The pilot needs also to use additional information relating to additional parameters of various kinds that are displayed separately, and then needs mentally to summarize all that information before undertaking appropriate piloting actions. This does not contribute to simplifying the pilot's task, and the pilot is often called on to take decisions extremely quickly in order to avoid endangering the crew and/or threatening the integrity of the aircraft. Amongst those additional parameters, there are parameters that are essential to piloting, such as, for example, the speed of rotation of the rotor.
The speed of rotation of the rotor needs to be monitored just as much as the monitoring parameters that are processed by the first-limit indicator. Too great a difference between the real speed of rotation of the rotor and the normal speed of rotation of the rotor, referred to as the “reference” speed of rotation, can lead to severe and irreversible consequences for the aircraft.
During certain stages of flight, when the speed of rotation of the rotor departs from the reference speed of rotation, the pilot acts on the collective pitch of the blades. Thus, when the real speed of rotation is too fast, the pilot increases the collective pitch to brake the movement of the rotor. Conversely, when the real speed of rotation of the rotor is too slow, the pilot decreases the collective pitch in order to increase said real speed of rotation of the rotor.
These piloting operations are not without danger insofar as there exist physical limits on the speed of rotation of the rotor that must under no circumstance be crossed. Thus, a maximum speed of rotation is defined for the rotor. In addition, beyond the physical limitations that are associated with the main gearbox or the rotor hub, there are stalling limits that must not be exceeded. Thus, a minimum speed of rotation is defined for the rotor at the outlet from the main gearbox. Consequently, the real speed of rotation of the rotor must remain within a range that is defined by maximum and minimum speeds of rotation.
Exceeding the maximum speed of rotation corresponds to crossing a bottom limit position for the collective pitch and to a speed of rotation of the blades that is greater than that imposed by the main gearbox. This is made possible by incorporating a free-wheel mechanism, known per se, in the main gearbox. This results in an interruption in the transmission of mechanical torque from the power plant and the main gearbox. Specifically, the pilot is then in a situation in which the speed of rotation of the rotor is no longer under control. Piloting the aircraft can then become very difficult or even impossible. The pilot is also confronted with a major risk of damage to mechanical parts and under the best of circumstances a maintenance operation will be required.
When the transmission of mechanical torque is interrupted between the power plant and the main gearbox, without exceeding the maximum speed of rotation of the rotor, the pilot is in a situation in which the collective pitch has crossed a threshold referred to as the “desynchronization” pitch. Such a situation is compatible, for example, with a stage of flight known as “autorotation” that a pilot may engage under certain circumstances, and in particular in the event of losing power from an engine. The value of the desynchronization pitch is independent of the maximum and minimum speeds of rotation of the rotor.
Crossing the minimum speed of rotation in the slowing direction corresponds to crossing a top limit for setting the collective pitch and to a speed of rotation for the blades that is slower than that imposed by the main gearbox. This results in an increased power demand on the turbine engine, which can result quite quickly in reaching an operating limit that leads to the loss of said engine. Under all circumstances, the pilot runs the risk of being confronted with insufficient or failing lift from the rotary wing.
In the particular circumstance of an aircraft having at least two turbine type engine units, failure of one of said engines can be compensated for some given duration by using the other engine. During this given duration, even if short, the above-described problem remains in full. The two turbine engines are said to be out of alignment. The pilot then has an indication displayed in the first-limit indicator that corresponds to a position for the collective pitch that is referred to as the “refuge” pitch in order to optimize use of the still-functioning engine.
Document WO 2006/081334 discloses an aircraft flight indicator for showing the power margin available on at least one aircraft engine as a function of flying conditions. The indicator described displays a plurality of parameters, and in particular a first parameter having a highest normalized value and a second parameter having a normalized value that is closest to its normalized limit (cf. FLI). Those indicators serve to inform the pilot about the speed of rotation of the rotor, the maximum and minimum values for the rotor speed, and the speed of rotation of the engine turbine. Provision is also made for piloting at a speed of rotation for the rotor that is different from the reference value.
Document US 2001/044679 also describes an aircraft flight indicator for showing the power margin available on at least one aircraft engine as a function of flying conditions. The indicator displays the parameter that is closest to its limit (FLI), and when the speed of rotation of the rotor drops below a determined limit, it informs the pilot of the correction to be made to the collective pitch.